Add a common debug flag envvar

The debug output in kalastack seemed really useful:
https://github.com/kalamuna/kalastack/blob/2.x/Vagrantfile#L112

Thought it might be nice to have this extra provisioner output off by default, but enabled with a single command:
https://github.com/patcon/kalastack/blob/develop/Vagrantfile#L109
https://github.com/patcon/kalastack/blob/develop/Vagrantfile#L121

PR pending